In recent years, there has been an increase in demand for metal resources. Among them, demand for noble metals has also increased as noble metals are used in e.g. electronic devices and various catalysts although reserves of them are scarce and supplies are limited. Therefore, the need for them to be effectively used and recovered has increased. One noble metal, Ruthenium, has also been used for wider applications including as a material for hard disks and as a catalyst in the production of hydrogen, and it has become important to improve techniques for recovery of ruthenium from waste fluid resulting from the production of such products or from the metal-dissolving solution generated during the recycling of industrial products.
In addition, as it is considered that radioactive elements are discharged as a result of nuclear plant accidents through subterranean routes to pollute the ocean, the removal of such pollutants is now critical. The radioactive elements discharged include radioactive ruthenium, and removal of the radioactive ruthenium from soil or sea water and/or purification thereof is also critical.
In such situations, ruthenium is usually present as an ion in an aqueous solution thereof and the recovery of ruthenium from or purification of ruthenium-containing water requires selective adsorption or filtration of ruthenium, or a treating agent which selectively reacts with ruthenium. In particular, the removal of radioactive ruthenium from and/or purification of radioactive ruthenium-containing water requires an extremely high removal performance, a simple removal apparatus, and a safe and inexpensive adsorbent and/or treating agent.
Many methods for recovering and/or removing noble metals including ruthenium have been previously proposed. For example, Patent Literature 1 describes a method of heating a material, on which a noble metal such as rhodium, palladium and ruthenium are supported, to allow the noble metal to be absorbed into a material having a perovskite crystal structure. Although this method is preferable for the recovery of a useful metal from a catalyst used in the treatment of automobile exhaust gas, the method requires a heating process at 1000° C. or higher, and thus it is difficult to adapt said method to applications in which a noble metal ion present in water is recovered and/or removed from the water.
Patent Literature 2 shows a method of allowing a metal ion to be adsorbed on an ion exchange resin or a chelating resin in an aqueous solution. Such a method of absorbing a metal ion with an ion exchange resin or the like provides only a limited removal of ruthenium per volume of an adsorbent due to the small number of adsorption sites in the adsorbent. This drives the cost required for the adsorbent up, and thus, in cases where a large amount of ruthenium-containing water is treated, the cost for the adsorption treatment of ruthenium increases, compromising the economic desirability of such a situation.
Patent Literature 3 shows a method of removing radioactive palladium or radioactive ruthenium from aqueous solution thereof by using a metal ferrocyanide. Although this method is preferable in that the ions in the aqueous solution can be removed, the ferrocyanide may decompose at high temperatures, generating harmful hydrogen cyanide. In addition, the removal efficiency is not sufficiently high, and moreover, decomposition may be caused in aqueous solutions with a high pH, allowing adsorbed radioactive ruthenium to be released and resulting in a lower removal efficiency. This complicates the management of the adsorption treatment system, and necessitate the treatment of cyanide-containing waste water, for safety.
Particularly in cases where the target for removal is, for example, water used for washing soil polluted with a radioactive element or sea water polluted with a radioactive element, such a target usually has a low concentration of ruthenium and typically requires large scale treatment, and thus the removal of the radioactive elements requires an extremely high removal performance in a small quantity, and a reduction in cost. With regards to removal efficiencies, e.g. for radioactive elements including strontium scattered by the explosion of a nuclear reactor, the final removal efficiency after purification must be 99% or higher or 99.9% or higher, because of the seriousness of its impact.
Another significant problem in the treatment of waste water from a nuclear reactor is the possible presence of ruthenium in various structures. As shown in Non Patent Literature 1, the structure of ruthenium is believed to change depending on the type of aqueous solution with which it has come into contact during nuclear fission from radioactive uranium. Ruthenium may be present in various structures depending on conditions such as the temperature of the aqueous solution thereof and chemical species dissolved therein, and a high removal performance is required for respective structures.
It is considered that a removal efficiency similar or comparable to that for radioactive strontium is necessary for radioactive ruthenium. Even assuming that the removal efficiency is 60% per cycle of treatment, 40% is discharged, and, for the removal of at least 99%, five or more cycles of treatment must be performed; however, this is significantly inefficient for applications in which a large amount of polluted water is treated. If a material which provides a removal efficiency of 95% or higher can be obtained, the discharge ratio after one cycle of treatment is at most 5%, and a removal efficiency of 99.75% or higher can be achieved in two cycles of treatment. Even in cases where a secondary treatment facility with e.g. a reverse osmosis membrane is provided downstream of the primary treatment with an adsorbent, the achievement of a high removal efficiency of, for example, 95% or higher at the primary treatment stage will significantly reduce the load applied to the step of the expensive secondary treatment, and this situation is very preferable in terms of cost-efficiency. Accordingly, this will require an adsorbent having a removal efficiency of 95% or higher for ruthenium cations which are expected to exist at a particularly high content, and having a removal efficiency comparable thereto for other ion species such as ruthenium complex ions and ruthenate ions.
Such an adsorbent needs to be capable of treating a large amount of water, and therefore needs to have sufficient stability for extensive treatment. For such a material, water-insoluble materials such as metal oxides are preferred.
In terms of safety, the use of an adsorbent or treating agent which induces environmental pollution is not preferable. Raw materials to be used need to be inexpensive, and moreover it is not preferable that their production involves the volatilization of an organic solvent, or the consumption of a massive amount of energy such as calcination at high temperature, and the process used to produce such materials should be simple.
As described above, although there is a high demand for the replacement of conventional methods for removing or recovering ruthenium by a simple and safe method using an inexpensive adsorbent or treating agent with a high removal efficiency, an adsorbent or treating agent satisfying performance requirements including 1) ability to remove various ion species and further 2) a high removal efficiency of 70% or higher, preferably 95% or higher, for ruthenium cations, as well as safety and low cost, has not yet been obtained.